The invention relates to 3-terminal bidirectional switching power supply (SPS), particularly for power supply systems requiring high output power, high efficiency, high reliability and very small size. One device operates as DC/DC, DC/AC, AC/DC or AC/AC converter. A power transformer is eliminated in order to minimize EMI, RFI, power loss, size, weight, cost, etc. Short circuit protection is inherent The SPS can be employed in a simple SPS, switching power amplifier, uninterruptible power supply (UPS), programmable converter, line conditioner, AC voltage converter, frequency converter, power factor correction circuit etc.
Conventional SPSs perform multiple conversions in order to obtain a desired AC output signal. The line voltage is rectified. The rectified voltage is stored in one or two capacitors and converted into high frequency AC signal which is transformed. The transformer output signal is rectified and voltages of opposite polarities are stored in two capacitors. The capacitor voltages are converted into the AC output signal which has the desired waveform. The SPSs employ the isolation transformers whether or not line isolation is required.
Charging the capacitors causes inrush current and surge currents every peak of the AC voltage. Pulse width modulation (PWM) is used which results in numerous problems including high circuit complexity, stability problems and significantly higher effective switching frequency. Numerous interference suppressors and protection circuits are inevitable. Power factor correction circuit is employed which further increases complexity and reduces efficiency.
Power transformers may be the worst components used in electronics, their employment demands specific considerations. For instance, the transformers demonstrate huge EMI and RFI, poor efficiency, leakage inductances, parasitic resistances, resonant elements, frequency limitations, flux symmetry problems, nonlinearities and saturation, bidirectional feedback, inability to provide DC signal, large size and weight, high cost. The imbalance in operating flux level is reduced by a series coupled capacitor which must sustain AC voltage and high currents. The transformers have low impedance output, wherein short circuit protection is inevitable.
UPS systems provide AC power during commercial power fluctuations and interruptions. An UPS comprises a charger, battery and inverter. The charger rectifies the line voltage and charges the battery. The inverter converts the battery voltage and provides the AC power. A continuous UPS system is coupled in series with line. The charger must be able to deliver enough power to drive the inverter at full load, maintain the charge on the battery and handle momentary overloads. A forward transfer UPS system is coupled in parallel with line and delivers AC power only during the power failure. A bidirectional transfer switch connects the critical load either to line or inverter output.
An exemplary approach to a high efficiency bidirectional SPS with AC or DC output is disclosed in the abovementioned "Switching Power Supply" U.S. Pat. No. 4,803,610 dated Feb. 7, 1989, by the same inventor. The SPS employs a minimum number of switches and no transformer. For instance, the FIG. 3 embodiment disclosed in U.S. Pat. No. 4,803,610 employs three switches and single inductor for converting a positive and negative voltages into AC or DC output signal and for transferring power from the output to the input sources.